In the production of hydrocarbons from marine oil and gas deposits, a fluid communication system from the sea floor to the surface is required. Such a system usually includes multiple conduits through which various fluids flow between a subsea well or pipeline to a surface facility. The multiple conduits for communicating with a surface facility typically include subsea trees, manifolds, production and export flowlines, buoys and riser systems.
One method for producing hydrocarbons from marine oil fields is to use a fixed facility attached to the seafloor, however, known fixed facilities can be enormously expensive. A lower cost approach for producing from marine oil fields involves the use of floating facilities or floating vessels. Floating vessels present additional challenges as they can undergo a variety of movements in an offshore environment and are exposed to rapidly changing and unpredictable surface and sub-surface conditions. In particularly extreme weather conditions, it may be necessary for the floating vessel to disconnect from its associated production flowline and riser system. Further, the disconnected riser system must be storm-safe while disconnected from the floating vessel.
Connections between the riser system and the floating vessel can be direct or indirect. In deep water, direct connection of single or bundled risers to a floating facility is feasible but generates constraints: 1) the riser must be installed after the floating facility is on location, which is usually on the critical path for planning purposes; 2) disconnection takes a long time and is not feasible under adverse weather conditions; and 3) the motion of the floating facility imposes stress on the risers which can be detrimental in terms of riser fatigue. Other direct connection systems have been proposed to support risers such as cylindrical buoys with or without ballasting elements, and riser towers, which may not be economical for small to medium reserves fields with a small number of risers.
Indirect connection of single or bundled risers to a floating facility is a preferred approach. Common industry practice is to accommodate vessel rotation about a fixed riser system by means of a turret and swivel assembly, which may be internal or external to the floating vessel. Such marine riser systems typically terminate in a buoy, which is designed to interface with the floating vessel. While disconnected from the floating vessel, horizontal loads from the risers are typically unbalanced. Industry practice has been to use a buoy anchor line, tether or mooring (tag line) to maintain buoy response within a prescribed vertical envelope. In deepwater, tag lines, which are usually anchored to the seafloor, become very expensive as the length of the tagline can be in the order of 7,000 to 10,000 ft. or more.
The aim of the present invention is to provide an alternative form of riser system in which the above mentioned problems are overcome or in the very least alleviated.
The invention in its preferred embodiments provides a low cost marine riser system for the recovery of hydrocarbons from the seafloor to a floating vessel. The riser system terminates in a disconnectable buoy that can be rapidly connected and disconnected to a floating facility, in the event of unplanned weather conditions. Further the riser system is capable of providing sufficient balanced tension to the disconnectable buoy to maintain the disconnectable buoy within a prescribed horizontal and vertical envelope while the buoy is disconnected from a floating vessel, without the use of additional tag lines.